An alkyl glycoside is a sugar derivative surfactant which is less irritating than other surfactant. Also, though it is a nonionic surfactant, alkyl glycosides form a stable foam per se, and furthermore, exert a foam-stabilizing effect on other anionic surfactants. These characteristics make alkyl glycosides highly noteworthy.
Although alkyl glycosides as novel surfactants have the above-mentioned noteworthy characteristics, it is quite difficult to produce them in the form of a commercially useful product. A most serious problem in the production of alkyl glycosides is that various procedures during the production process thereof frequently cause deterioration of the hue of the product. Furthermore, the alkyl glycoside thus produced suffers from deterioration of hue upon storage with the lapse of time.
In order to produce an alkyl glycoside which has a good hue sufficient to be useful for application in a commercial product, it has been found necessary to decolor alkyl glycoside that has been obtained by reacting a sugar with an alcohol. However, the use of hydrogen peroxide (which is commonly known as an effective decoloring agent) in the decoloring of the alkyl glycoside is accompanied by other new problems, such as, for example, deterioration with respect to odor, caused by rancidity and aldehyde smells evolving upon storage, and deterioration in the hue with the lapse of time.
Accordingly, there have been attempts to improve the hue of an alkyl glycoside. For example, JP-A-59-65098 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") (corresponding to U.S. Pat. No. 4,704,453) discloses a process wherein a reducing sugar is acetalized in the presence of an alkali borate in an amount equivalent to, or exceeding, an acid catalyst; JP-A-59-139397 discloses a process wherein acetalization is conducted in the presence of an acid catalyst and a reducing agent; and JP-A-60-1196 discloses a process wherein acetalization is conducted with the use of a perfluorosulfonic acid resin as a catalyst. Furthermore, European Patent 0132046 reports a process wherein a catalyst is neutralized with an organic base after the completion of an acetalization reaction; JP-A-58-194902 (corresponding to U.S. Pat. No. 3,565,885) discloses a process wherein a residual alcohol is removed by using a thin film evaporator; U.S. Pat. No. 3,450,690 discloses a process wherein alkali labile color-producing bodies are removed by treating with an alkali; and JP-A-47-16413 (corresponding to U.S. Pat. No. 3,565,885) discloses a process wherein said removal is conducted with the use of a hydroxyl type anion exchange resin. Furthermore, JP-A-62-192396 discloses a process wherein a viscosity depressant is added upon the separation of the formed alkyl glycoside from the unreacted and recovered alcohol, since the high viscosity and poor heat stability of the alkyl glycoside causes particularly serious deterioration of the hue.
In addition, JP-A-1-290692 discloses a process for improving the color of a glycoside composition which comprises contacting a glycoside composition containing colored human with a hydrogen source (for example, hydrogen or sodium borohydride), but the product is still deficient in hue (see Comparative Example 4 below).
However, none of the foregoing methods can provide an alkyl glycoside which shows a satisfactory hue when applied to a commercial product in practice.
JP-A-61-33193 (corresponding to U.S. Pat. No. 4,557,729) discloses bleaching the finally obtained alkyl glycoside with hydrogen peroxide and sulfur dioxide. In this case, however, the odor of the alkyl glycoside immediately after the treatment is inferior to that prior to the treatment. Furthermore, the alkyl glycoside thus obtained suffers from deterioration in the hue and odor with the lapse of time upon storage. Thus, this is not an effective measure.
As described above, it is important that the hue and odor of an alkyl glycoside which has been decolored with hydrogen peroxide be stable upon storage. Since this problem has never been overcome so far, it has been urgently required to establish a solution thereto.